Answer:
If a hiker had not screwed on the cap of a bottle before returning to sea level, the decrease in external pressure would cause the air pressure inside the bottle to be greater than the pressure outside. This difference in pressure could cause the bottle to expand or even burst if the material is not strong enough to withstand the pressure difference.
The degree of expansion or deformation would depend on a few factors, including the volume of air trapped inside the bottle, the size and shape of the bottle, and the material properties of the bottle. If the bottle is made of a strong and rigid material like glass, it may not expand or deform significantly. However, if the bottle is made of a more flexible material like plastic, it may bulge or deform under the increased pressure.
In general, the bottle would look swollen or bulging, and there may be visible deformation or warping of the material. The cap may also be forced off due to the increased pressure inside the bottle. Additionally, if the contents of the bottle are under pressure (such as a carbonated beverage), the pressure difference could cause the liquid to foam or bubble out of the bottle.
It's important to note that this scenario is not recommended as it could be dangerous and result in injury from shattered glass or flying debris. It's always best to securely cap any containers before transporting them to different altitudes or pressure conditions.